In PR #16295 one occurrence of this was changed, but a few more remained
in the codebase. This commit fixes the other occurrences so that we
don't use the deprecated way of creating custom events anywhere anymore.
According to MDN, see https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/initCustomEvent,
using the `CustomEvent.initCustomEvent` method is deprecated and the
`CustomEvent` constructor should be used instead.
Extends d9bf571f5c.
The size of the `web/ui_utils.js` file has increased over time, as more code has been added to (or moved into) that file. To reduce its size slightly, this patch moves the event-related functionality into a separate file.
